As more states are poised to legalize medicinal marijuana, it's looking like dope is playing a larger role as a cause of fatal traffic accidents.

Columbia University researchers performing a toxicology examination of nearly 24,000 driving fatalities concluded that marijuana contributed to 12% of traffic deaths in 2010, tripled from a decade earlier.

NHTSA studies have found drugged driving to be particularly prevalent among younger motorists. One in eight high school seniors responding to a 2010 survey admitted to driving after smoking marijuana. Nearly a quarter of drivers killed in drug-related car crashes were younger than 25. Likewise, nearly half of fatally injured drivers who tested positive for marijuana were younger than 25.

A National Highway Traffic Safety Administration study found that 4% of drivers were high during the day and more than 6% at night, and that nighttime figure more than doubled on weekends.

Colorado has seen a spike in driving fatalities in which marijuana alone was involved, according to Insurance.com. The trend started in 2009 — the year medical marijuana dispensaries were effectively legalized at the state level.

NHTSA and the National Institute on Drug Abuse are now in the final months of a three-year, half-million-dollar cooperative study to determine the impact of inhaled marijuana on driving performance. Tests observe participants who ingest a low dose of THC, the active ingredient in marijuana, a high dose and a placebo to assess the effects on performance, decision-making, motor control, risk-taking behavior and divided-attention tasks.

The study is being performed using what NHTSA calls "the world's most advanced driving simulator," the University of Iowa's National Advanced Driving Simulator, which was previously used to study the effects of alcohol on driving.